The lithium ion battery is an ideal power source due to high monomer voltage, small size, light weight, no memory effect, no pollution, small self discharge and long cycle life. In actual use, in order to obtain higher discharge voltage, at least two monomer batteries are generally connected in series to form a battery pack. At present, the lithium ion battery pack has been widely applied to various fields such as notebook computers, electric bicycles, standby power sources, etc. Meanwhile, the lithium ion battery pack is the best power source for electric vehicles in the ascendant.
The requirements of the lithium ion battery for charging and discharging are completely different from reversible electrochemical reaction type batteries such as lead-acid batteries. Because the lithium ion battery packing application technology, the system integration key technology and key part and product researches seriously fall behind the development of the lithium ion battery, the problems of overcharge, over discharge, over temperature and over current occur after battery packing, so that the service life of packed lithium ion batteries is greatly shortened, the safety is greatly reduced, and even terrible accidents such as combustion, explosion, etc. occur. These become main problems restricting the development of the lithium ion battery industry, and also are technical bottlenecks for the development of the current energy saving and new energy source vehicle industry.
Until today, some good achievements about the electric vehicle technology in China have been obtained in the fields of vehicle power batteries, motors, electric drive, etc. Although the vehicle power battery technology is not mature, the development speed is not slower than that of developed countries. When a battery monomer is measured, various displayed indexes basically reach the design requirements. However, when the battery monomers are really integrated into a power assembly or integrated on a complete vehicle, the results are quite different from those obtained in monomer measurement. The vehicle battery assembly is not formed as simple as connecting the monomer batteries in series or in parallel one by one. The integration of dozens of even hundreds of batteries on a vehicle is a high and new technology even around the world and is not as easy as it appears, and not too many units or individuals have the ability to solve the difficulty. The lithium ion battery system mainly comprises a battery system, a charging system, a discharging system and a maintenance management system. The lithium ion battery system is a high technology integration system covering various technical fields and industries.
Traditional Charging Method                1) Serial charging method: At present, the charging of the lithium ion battery pack is generally serial charging, because the serial charging has simple structure, low cost and easiness in realization. However, the monomer lithium ion batteries have difference in capacity, internal resistance, attenuation, self discharge and other properties. 100 lithium ion batteries with 100 Ah discharge capacity are serially connected to form a battery pack. If 99 monomer lithium ion batteries have 80 Ah charges and one monomer lithium ion battery have 100 Ah charges before packing, when the battery pack is used for serial charging, the monomer lithium ion battery with 100 Ah charges is firstly charged fully to achieve overcharge protection voltage. In order to prevent the monomer lithium ion battery from being overcharged, the battery management system will cut off the entire serial charging circuit, so that the rest 99 batteries are not fully charged, and the discharge capacity of the whole battery pack is only 80 Ah. The serial charging has the defects of wasting the capacity of the battery pack due to insufficient battery pack charging or having the risk of battery explosion due to overcharge.        2) Battery management system and charger coordinated and matched serial charging method: The battery management system is most comprehensive equipment for knowing about the properties and the status of the batteries, so that the information of the batteries can be known by the charger in real time by establishing connection between the battery management system and the charger. Some problems occurred during the charging of the batteries can be solved more effectively. However, the charging method still belongs to serial charging, so the charging method carries on all defects of serial charging.        3) Parallel charging method: In order to solve the problems of overcharge and insufficient charging of certain monomer batteries in the battery pack, the parallel charging method is generated. However, the parallel charging method requires multiple low-voltage large-current charging sources to charge each monomer battery, so that the parallel charging method has the defects of high charging source cost, low reliability and low charging efficiency. Assuming that the vehicle adopts a three-phase AC motor and the input DC voltage of an inverter is 288V, power is supplied by a lithium power battery pack including 80 monomer batteries with voltages 3.7V. The parallel charger must comprise 80 completely isolated DC sources with 200 A constant output current and 3.7V constant output voltage. The completely isolated constant-current constant-voltage DC sources must be properly connected and disconnected, and the complexity is almost unrealizable in practice.        4) Serial large current plus parallel small current charging method: Because the above three charging method have problems, another charging method most suitable for a high-voltage battery pack especially an electric vehicle battery pack is developed, i.e. the battery management system and charger coordination and matching, serial large-current, constant-voltage current-limiting parallel small-current charging method is adopted. Although the charging method integrates the advantages of the above three charging method, all the defects of the three charging methods are completely carried on.        